To achieve a fault-tolerant quantum computing system, it will be necessary to further improve single qubit gate fidelities to reduce the overhead for quantum computing. In this talk, we show that by employing GRAPE computed optimised control pulse shaping scheme and other calibration techniques, we can improve average Clifford gate fidelities for silicon quantum dot spin qubits by an order of magnitude, to 99.96% (equivalent coherence time of 9.4ms) experimentally, the highest up to date record in silicon quantum dot qubits.

Long coherence times can be achieved by using the spin states of the rare earth ion erbium. These spin states are accessible with optical transitions, which provides possibilities for long range coupling of these states. We investigate the magnetic properties of single erbium ions in silicon in order to identify suitable ions for quantum communication. Our results show that the individual ions have highly anisotropic Zeeman splittings which vary from ion to ion.

In principle, quantum computers that exploit the nature of quantum physics can solve some problems much more efficiently than classical computers can. Motivated by the tremendous scalability of classical silicon electronics, a group of us at University of Wisconsin-Madison are working to build a large-scale quantum computer using silicon/silicon-germanium quantum dots, constructed using technology similar to that used to build current classical computers.

The Quantum Hall Effect (QHE) was discovered in 1980 and many works have been published since then related to the Physics of this phenomenon. However, there still exists misconception about its microscopic picture in the form of the dissipationless 1D current-carrying edge-states. Thus, I will present scanning probe microscopy measurements conducted at the von Klitzing department (MPI Stuttgart), which have demonstrated the actual current distribution inside the Hall sample during QH plateau [1].

Phosphorus donor devices in silicon fabricated using STM hydrogen resist lithography are typically fabricated in a 2D design using a single lithographic layer. However in order to achieve quantum error correction with a surface code architecture we need to be able to fabricate devices in three dimensions using multiple lithographic layers [1]. Here I will present on the fabrication challenges associated with transitioning from single layer devices to a multilayer structure and the techniques we used to overcome these challenges, allowing for the fabrication of high quality multilayer devices.

I will present some recent breakthrough theoretical results to identify and suppress errors across a quantum computer in an efficient and practical manner. I will also present preliminary results on the implementation of these methods on a 10 qubit ion trap quantum computer.

Qubits encoded in the electron spin states of gate-defined quantum dots are promising because of their long coherence time. Recent experiments have realized both single qubit operations with fault-tolerant fidelity [1-2] and two qubit logic gates [3-5]. For single qubit gates, randomized benchmarking has emerged as a popular characterization tool. However, for two-qubit gates it has so far only been applied to a few qubit implementations.

In order to achieve fault-tolerant quantum computation we must meet a very demanding set of requirements, including a very low error rate for qubit readout, much less than 1%. The error rate for ancilla qubit readout includes decoherence of data qubits that occurs during the readout time, which means that readout must be not just accurate, but also much faster than the qubit T2 time.

Two high-profile results on coupling the spin of gate-defined quantum dots to superconducting microwave resonators have recently been published. I will review the 2018 papers from groups at Princeton ["A coherent spin-photon interface in silicon", doi:10.1038/nature25769] and Delft ["Strong spin-photon coupling in silicon", doi:10.1126/science.aar4054].

The 123-Sb atom is a group-V element with a nuclear spin quantum number of 7/2, resulting in an 8-dimensional Hilbert space. This atom can be implanted in a silicon Metal-Oxide-Semiconductor structure, and its quantum state can be controlled using the same infrastructure that has been proven to yield high-fidelity control and single-shot readout on the 31-P donor.

Strain is a common ingredient in solid-state systems which has been widely used to enhance electronic devices performances, both electrically and optically, via a change in the band structure of materials. Consequently, strain also induces a change in the properties of single dopants, and schemes have now emerged to use strain to define or tune qubits for quantum technology purposes. Ensemble measurements of donors in strained silicon have been carried out, but yet to date no experiment down to the single atom level has been performed.

Future quantum technologies require quantum bits that remain coherent over long time scales, a goal recently achieved for electron spins in some semiconductors. Because of their strong spin-orbit coupling, hole-based qubits have attracted interest to achieve long-distance coupling, and to build hybrid quantum systems and spin/photon interfaces. However, it is not known if spin-orbit coupling of holes is compatible with long coherence times, since to date, experimentally reported values are too short for most envisioned applications.

I will present our method to experimentally realize high-fidelity single qubit operations for a qubit encoded in two electron spins in GaAs quantum dots by iterative tuning of the all-electrical control pulses. We find an average gate fidelity of F = 99.5% and determine gate leakage out of the computational subspace to L = 0.04%. These results demonstrate that high fidelity gates can be implemented even in the presence of nuclear spins.

Minimizing the number of physical gates required for control of semiconductor spin qubits is an important problem. A promising strategy is to use rf reflectometry with “gate-based” charge sensing for qubit readout. Studies to date have focused on charge sensing performance in the regime of weak rf driving where the response is linear, but when driven strongly the admittance of a quantum dot saturates to a constant ac current due to Coulomb blockade.

We present a donor based quadruple quantum dot device, designed to host two singlet-triplet qubits fabricated by scanning tunnelling microscope lithography, with just two leads per qubit. The design is geometrically compact with each pair of dots independently controlled via one gate and one reservoir. The reservoirs both supply electrons for the dots and measure the singlet-triplet state of each qubit via dispersive sensing. We verify the locations of the four phosphorus donor dots via an electrostatic model of the device.

Singlet-triplet qubits are one particularly successful implementation of spin qubits that has recently demonstrated improved immunity to charge noise by applying intense magnetic field gradients. However, large magnetic field gradients allow for relaxation between qubit states during measurement, reducing readout visibility to almost zero. Here we present a new technique that is robust against these relaxation pathways and enables working above magnetic field gradients of 400 MHz, a regime that unaccessible using previously studied methods.

A challenge for modelling quantum dot spin qubits in silicon is the quantitative estimation of the exchange energy J over a wide range of electrostatic potentials. Approximate methods such as Heitler-London and Hund-Milliken are often applied to GaAs dots, but break down in silicon due to the larger effective mass of the electron. I will discuss our progress towards building a simulation toolbox for exchange energies based on configuration interaction (CI) methods commonly used in quantum chemistry.

Qubit state readout is an essential step for quantum computation and must introduce minimal errors if full error correction is to be achieved with a surface code architecture. The lowest errors during readout will only be possible if the readout is not only high fidelity, but also high bandwidth. Typically, for electron spin qubits in Si-P donor based systems, a single electron transistor (SET) is used as a charge sensor to perform single-shot electron spin readout.

Heavy hole spins in quantum dots are promising candidates for spin qubits. This is because holes have reduced hyperfine coupling to nuclear spins, allowing long spin coherence times [1], while the enhanced spin-orbit coupling of holes enables fast all-electric spin manipulation via EDSR [2]. However, challenges in device fabrication and complexities in theory have limited the number of studies using hole-based devices. In this talk we discuss our recent progress in silicon-based hole quantum dots.

Advancements in Quantum-dot Cellular Automata (QCA) fabrication have posed this technology as an alternative to CMOS for general-purpose computing. Reliable simulation of QCA circuits requires computation of the ground state and dynamics of the complete quantum mechanical formulation, which becomes computationally infeasible with increasing problem sizes. In this paper, we present an embedding algorithm which maps a QCA circuit to a D-Wave Quantum Annealing Processor to allow for efficient ground state computation.

Many processes in quantum information science rely on nonlinear optical interactions. In this talk, we will present our progress in Kerr nonlinear plasmonics. I will first discuss the advantages and challenges of using plasmonics for nanoscale nonlinear applications. In order to quantify the ultimate nonlinear performance and compare different plasmonic waveguides, we propose a versatile figure of merit. We also provide a deep understanding to the ultimate nonlinear performance.

In this talk we will discuss the progress in precision donor qubits in silicon. We will demonstrate the successful integration of a microwave transmission line onto two few electron double donor dot devices in silicon fabricated with the atomic precision of a scanning tunneling microscope (STM). The transmission line is used for magnetically driven Electron Spin Resonance (ESR) and a DC-coupled SET charge sensor is employed for single-shot spin readout. In the first device we use ESR spectroscopy to identify the characteristic hyperfine spectra of a single donor 1P and two-donor 2P dots.

As the size of microelectronic devices approaches fundamental limits, their performance is strongly influenced by the local environment inside the device, such as electric field and strain. Here we present the effect of applied strain on single erbium ions in a silicon transistor. This result, in conjunction with the Stark effect detection demonstrated previously, can be utilised for non-destructive 3D imaging of the local strain and electric field in nano-transistors, using the single erbium ions as atomic sensors.

We investigate spin and charge dynamics of a quantum dot of phosphorus atoms coupled to a radio-frequency single-electron transistor (rf-SET) using full counting statistics and inverse counting statistics.
We show how the magnetic field plays a role in determining the bunching or anti-bunching tunnelling statistics of the donor dot and SET system. Using the counting statistics we show how to determine the lowest magnetic field where spin-readout is possible. We then show how such a measurement can be used
to investigate and optimise single electron spin-readout fidelity.

First, I will present results on semiconducting Ge/Si core/shell nanowires: In double quantum dots, we observe shell filling of new orbitals and corresponding Pauli spin blockade. In nanowires with superconducting Al leads we create a Josephson junction via proximity-induced superconductivity. A gate-tuneable supercurrent is observed with a maximum of ~60 nA. We identify three different regimes tuneable via backgate voltages: Cooper pair tunnelling, quasiparticle transport and finally full suppression of transport.

Singlet-triplet readout of two nearest neighbor quantum dots is an essential tool for scaling up quantum computing in silicon MOS systems [1]. Traditionally this parity readout is done in (2,0) charge configuration of a double quantum dot systems where triplet states are blockaded in (1,1) state due to Pauli exclusion principle. This difference in the charge configuration can be detected with a charge sensor only if the two dots have sufficient differential capacitance to the sensor.

Light combines the ability to carry quantum information in ambient conditions with a large information capacity, making it ideal for building quantum networks. However, due to the probabilistic nature of linear-optical entangling operations, it remains an outstanding challenge to grow such networks. Historically, the goal of swapping entanglement over large scale networks motivated the development of "quantum repeaters'', based on quantum memories that can trap and release photons on demand to synchronise entangling operations.

We perform direct single-shot readout of the singlet-triplet states in exchange coupled electrons confined to precision placed donor atoms in silicon. Our method takes advantage of the large energy splitting given by the Pauli-spin blockaded (2, 0) triplet states, from which we can achieve a single-shot readout fidelity of 98.4 +- 0. 2 %. We measure the triplet-minus relaxation time to be of the order 3 s at 2.5 T and observe its predicted decrease as a function of magnetic field, reaching 0.5 s at 1 T.

Silicon Metal Oxide (SiMOS) based architectures are an excellent platform for single electron spin qubit systems. SiMOS systems possess long coherence times1, allow high fidelity control of electron spins1, and enable a two-qubit logic gate1. Current technology features spin control via electron spin resonance (ESR) and sensing is achieved via an on chip single electron transistor enabling single-shot reservoir spin readout. However, extending the SiMOS platform to a larger number of qubits will require an alternative readout mechanism.

The spin degree of freedom of an acceptor in silicon provides an attractive alternative to the well-established donor spin qubit in silicon due to the potential to electrically manipulate the quantum state. Here we discuss the modification of the spin-orbit coupling of a hole trapped by a boron site in a silicon transistor based on electrical field

The spin states of rare earth atoms such as erbium have long coherence times which makes them interesting for the use as quantum memories. These spin states can be accessed through optical transitions, with erbium having transitions at the convenient wavelength of 1540 nm. We use these optical transitions to study erbium’s spin states inside a silicon transistor in order to determine the local site symmetry as well as the crystal field splitting.

While a number of breakthroughs have been made in donor-based qubit systems, the small dipole moment of donor spins makes inter-connection of many qubits challenging. Acceptor spins, on the other hand, possess spin-orbit coupling, which offers a spin qubit with a large electric dipole moment. However, static and dynamic properties of acceptor spins have yet to be unveiled in experiments. Towards realisation of an acceptor qubit, we investigate spin relaxation and spin coherence for two different kinds of spin level configurations of boron atoms, in both single atom and ensemble measurements.

The nuclear spin state of a phosphorus donor in isotopically enriched silicon-28 (Si:P) is an excellent host to store quantum information in the solid state. The spin's insensitivity to electric fields yield a solid-state qubit with record coherence times, but also renders coupling to other quantum systems very challenging. I will show that, by coupling the phosphorus donor to an electron shared with an interface dot, a magnetic drive creates a strong electric dipole

Pulse optimisation has been implemented in system such as NMR to improve nuclear spin-readout fidelity. In this talk, we show that optimised microwave pulse calculated using GRAPE algorithm can improve our silicon spin qubit gate fidelities through simulation. Preliminary results of such optimised pulses that applied to our physical qubit will also be presented.

Silicon (Si) quantum dots (QD) have been among the most prominent candidates for implementing spin based qubits with a potential for scalability, due to their exceptional coherence times and industry standard fabrication process. To build a large-scale quantum computer with Si QDs, we must address any dot-to-dot variations that can cause randomness in qubit operations.

Electronics and optoelectronics technologies rely on the control of electric charge at the interfaces between active materials of solid-state devices. This behaviour is dictated by quantum mechanical phenomena unfolding at the nanoscale and depends strongly on the atomic-scale morphology of these systems.

Single implanted atoms in silicon are known to be extremely good individual qubits. A way to couple many of them together is to use a superconducting resonator, as extensively used for superconducting qubits.
In this presentation, I will present an intermediate step towards this objective: the theory of the readout of a single nuclear spin state, using a microwave cavity.

Micro- and nanomechanical elements are extensively studied due to their importance in force and mass sensing applications. To access their mechanical response, these vibrating elements are typically integrated into an electronic, electromagnetic, or optical environment. In cavity optomechanics, the interaction of a light field in an optical resonator with the mechanical degree of freedom goes beyond the sole readout functionality. Here, the light-matter interaction enables the manipulation of the mechanical state, manifesting itself e.g.

Single photon detectors are essential elements in the field of quantum optics. Amongst all the existing detector technologies, superconducting single photon detectors (SSPDs) based on WSi material turned out to be promising due to their high efficiency at near infrared wavelengths, fast recovery time, low timing jitter and low dark count rate. Moreover, the as-deposited WSi film is amorphous and highly uniform. The fabricated devices thus show high reproducibility and are more robust to the substrate defects.

Hybrid circuit QED is a key tool for readout and scaling of both semiconductor-based spin and topological quantum computing schemes. However, traditional approaches to circuit QED are incompatible with the strong external magnetic fields required for these qubits. We present previous work on superconducting CPW resonators and graphene SNS Josephson junctions that are engineered to survive parallel applied fields up to 6 T. We then combine these elements to realise a magnetic field compatible transmon qubit operating at 1 T.

At present the sensitivity of spin qubit measurements is not limited by any fundamental physics but, typically, by the noise floor of the first-stage amplifier. In the superconducting qubit community the Josephson Parametric Amplifier (JPA) has been developed to enable fast qubit readout, which has demonstrated quantum-limited noise performance. But the JPA is highly sensitive to magnetic fields and is not ideal for integration with spin qubit experiments. An alternative possibility is to implement a parametric amplifier by taking advantage of the nonlinear capacitance of a quantum dot.

Quantum cryptography requires a high-rate, true single-photon source in order to attain guaranteed security, while photon-based qubits offer the advantage of compatibility with quantum communication frameworks. We have developed a method of creating lateral p-n junctions in undoped GaAs wafers capable of producing single photons using a surface acoustic wave (SAW). In a piezoelectric material, a SAW consists of both an electrostatic potential and an elastic wave travelling parallel to the surface.

Single-electron pumps based on semiconductor quantum dots are promising candidates for the emerging quantum standard of electrical current. They can transfer discrete charges with part-per-million (ppm) precision in nanosecond time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to experimentally demonstrate high-accuracy gigahertz single-electron pumping in the regime where the thermal excitation of electrons, during the equilibrium charge capturing process, is the predominant error mechanism.

Classical conservative systems usually exhibit rapid dispersion of initial conditions – chaos – while the corresponding quantum equivalent system exhibits quasi-periodicity, localization, and tunneling through classically forbidden regions in phase space. How to reconcile these strikingly different behaviors has been the topic of active theoretical debate, but is accompanied by few experimental results. We propose an experiment aimed at realizing the real-time experimental observation of a single quantum system whose dynamics is classically chaotic – a periodically-driven nonlinear top.

Spins in silicon have proven to be a suitable platform for the development quantum technologies. Understanding and controlling spin coupling is now a formidable challenge to overcome. In this talk our most recent results will be reviewed, where atomic scale STM fabrication is combined with low temperature donor imaging to enable local measurements of quantum dots in the STM. This technique will allow manipulation and local read-out of donor states, beneficial from the understanding of qubit coupling in quantum computation to the topological properties of large scale complex arrays of spins.

Heisenberg exchange is a key process for entangling single spin qubits and defining logical spin qubits in silicon. However, the role of valley degrees of freedom in exchange is not yet experimentally understood. Here we spatially map the exchange interaction between a single donor atom and a single-electron quantum dot that can be positioned with sub-nm precision using a scanning tunneling microscope. Exchange is found to vary smoothly in space, due to the disorder-free interface for the dot.

An interface between a well-functioning, scalable stationary and a photonic qubit could substantially advance quantum communication applications and serve as an interconnect between future quantum processors. Qubits consisting of gate-defi ned quantum dots in GaAs are electrically controllable with high delity, whereas qubits that can realize bound exciton states are established as an optical interface. Here, I present a protocol to transfer the state of a photonic qubit to a gate-defined quantum dot single-spin qubit as well as to a two-spin qubit.

Microsoft Station Q at Delft and QuTech, Delft University of Technology, The Netherlands

When:

4pm Thursday 12 January 2017

Where:

CQC2T Conference Room Level 2, Newton Building, UNSW

Majoranas in semiconductor nanowires can be probed via various electrical measurements. Tunnel spectroscopy have revealed zero-bias peaks in the differential conductance. New observations include quantum superpositions of Majorana states leading, for instance, to a 4pi current phase relation or a fractional Josephson effect. When the existence of Majoranas is firmly established, the next challenge is to build Majorana qubits. We discuss the different qubit schemes and report on our first building blocks.

As quantum dot devices grow in complexity, more groups are adopting the use of RF probes to measure the state of their qubits due to the sensor’s compact real-estate, high measurement bandwidth and general performance under low frequency noise. This seminar is to provide a general overview of the RF probe, specifically in the case of RF reflectometry, as applied to gate-defined and donor-based quantum dot systems. The state of the art, as seen in literature, shall be covered while noting some technical subtleties involved in optimising the sensor.

High fidelity qubit state readout is one of the essential steps to achieve universal quantum computation. In this talk I will focus on spin state systems and what progress has been made to improve spin read-out over the past ten or so years as well as what further optimisations we have been investigating. Various parameters, such as device architecture and magnetic field strength, can be optimised to produce the highest read-out fidelities possible for a particular system.

In addition of being extremely successful platforms for spin qubits, silicon quantum dots can be operated as robust quantized current sources. These current pumps would provide a convenient realization for the emerging quantum SI ampere, which would be based on fixed elementary charge. Here, we study silicon quantum dot charge pump that can output 80 pA current with uncertainty of less than 30 ppm and show that the pumping dot can be manipulated with external electric confinement [1]. Electron counting is performed with nearby integrated charge sensor.

We have recently proposed [1] a new scheme to operate and couple Si:P spin qubits that does not require precise donor placement and spaces them apart allowing plenty of room for interconnects. Such a scheme relies on manipulating the electron charge state, and therefore care has to be taken in protecting the qubit from charge noise. In this seminar I will discuss how different sources of noise affect the performance of our quantum gates, and show that, by operating the qubits in regimes where they are protected from noise, fidelities compatible with quantum error correction are within reach.

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy has numerous applications in chemistry, biology, and materials science [1]. Most ESR spectrometers rely on the inductive detection of the small microwave signals emitted by the spins during their Larmor precession into a microwave resonator.

Thermal transport is an important physical phenomenon, and it has recently become even more relevant for the reduction of energy losses and the increase of efficiency in novel devices based on thermoelectricity [1]. Significant reduction of thermal conduction was recently achieved by coherent modification of phonon modes [2], with the help of periodic phononic crystal structures. However, currently the experimental studies have only been performed for two-dimensional (2-D) nanostructures.

Optomechanics deals with the interaction between electromagnetic radiation and mechanical objects. The optomechanical interaction is caused by radiation-pressure force, which was experimentally observed over a century ago. Modern interest in optomechanics is motivated from a few different directions: ultra-sensitive optical detection of forces, displacements and accelerations (e.g.

We put forward a hybrid approach in which optical cavities are applied to coupling qubits and electronic devices are used to readout. The key point is to guarantee the electronics compatible with photonics. We propose that using doping Phosphorus in silicon to form a conductive layer acting as electrodes and conducting wires in the cavity. The optical cavity could be used to transfer the photons to couple single atom qubits.

Optical addressing provides optical/electrical access to single erbium atoms in silicon. The next step is to look at its nuclear spin dynamics. In this presentation, I’ll show the recent experimental progress with more efficient readout at lower temperature, and an outlook for future work.

Faculty of Natural Sciences, Department of Physics, Imperial College London

When:

4pm Thursday 15 September 2016

Where:

CQC2T Conference Room Level 2, Newton Building, UNSW

The organic dye molecule dibenzoterrylene (DBT) in an anthracene crystal matrix is a promising
candidate for single photon emission. At cryogenic temperatures, this system presents a narrow
lifetime-limited transition at 785nm, with a quantum yield close to unity. Moreover, DBT
molecules have been shown to act as a mediator for photon-photon interactions, by inducing a
phase-shift on a passing photon when another photon is present. These features make DBT
molecules a powerful tool for quantum information purposes, including use as single photon sources

The distribution of tunnel events in a system can reveal a large amount of information about the system dynamics that may not be immediately apparent. The statistics of this distribution can be measured by counting the number of transition events within a certain time: This is known as full counting statistics (FCS). In this talk I will give an overview of full counting statistics and introduce a new technique known as inverse counting statistics (ICS), which can be used to obtain further information about the dynamics of the system.

Phosphorus donor nuclear spins in silicon have long coherence times and a small spatial footprint, making them an attractive candidate for a large-scale quantum processor. We present an architecture that takes advantage of the uniformity of donors and the resolution of hydrogen desorbtion lithography to implement an error corrected array using the 2D surface code. The difficulties of independent qubit control and tuning/trimming are avoided and the complexity of all quantum operations is distilled to careful loading and unloading of electrons.

Self-assembled quantum dot provides us a platform combined with highly indistinguishable single photon source and well defined spin qubit. In this talk, I will present the first experiment on generating Two-Particle-Three-Qubit type GHZ entanglement in this system. Based on this entanglement state, a photon state is teleportated to a spin of a quantum dot in 5 meters distance.

The acceptor spin is predicted to have a large electric dipole moment and a long coherence time in the well-controlled strain and electric field. These features are suitable for realization of spin qubits wired-up by a superconducting cavity. In this talk, I will present some preliminary works to evaluate the coherence time of acceptors and incorporate the electric field and strain with a high-Q cavity.

The interaction of photonic structures with single photon emitters at visible wavelengths is of great interest in fundamental quantum information processing and biological sensing. At room temperature, colour centres in diamond have shown great advantages over other solid state emitters in many experiments.

Abrupt dopant profiles and low resistivity are highly sought after qualities in the silicon microelectronics industry and, more recently, in the development of an all epitaxial Si:P based quantum computer. Previously, we have shown that increasing the dopant density by growing multiple layers is ultimately limited the formation of P-P dimers due to the segregation of dopants between multi-layers [1].

I would love you to join me at the opening of my latest exhibition, Schrӧdinger's Bird, which represents a unique art and science collaboration with the world renowned ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T). The exhibition encompasses a collection of drawings, paintings, animation and machinery that explores the heart of Quantum Physics and the new science of quantum computation which is the area of focus of this world-leading research group.

When: The exhibition runs from the 24th May to 12th June 11am - 5 pm daily;

Since the Kane proposal our understanding of the need for and methods of quantum error correction has developed significantly - motivating improved architectures for quantum computing, in particular based on the surface code. I’ll present an analysis of a novel scheme for implementing a surface code with donor spins in silicon using their dipolar interaction and a repeating mechanical motion.

Hydrogen-resist lithography on Si(100) has become a reliable tool to fabricate nano-scale circuits. Traditionally, our devices were constricted to a single 2D plane. Our technique can be adapted to the fabrication in 3D as well. In this talk I will discuss the requirements on alignment and surface quality. Finally, I will present results of two working 3D single electron transistors, one with a top gate and one with an additional single donor tuned by the top gate.

Quantum mechanics is often said to be a ‘strange’ theory: but what exactly is meant by this? Often, what is meant is the failure of our ability to apply certain classical notions to the atomic or molecular scale. I will discuss two such notions of classicality, i) the idea that objects have definite properties independent of measurement and ii) that uncertainty can be thought of as merely imperfect knowledge.

Measurement of a quantum system appears to create a discontinuity in its evolution, since superpositions are collapsed into eigenstates.

The Many World Interpretation (MWI) posits that this collapse is merely subjective, providing a new perspective on the apparent paradoxes invoked by incompatible measurements of the same quantum system. Solomon will discuss this approach and present results for a violation of a "Bell's inequality in time": the Leggett-Garg inequality.

Abstract: Magnetic molecules with magnetic bistability have represented the ideal workbench for the investigation of quantum effects in the magnetization dynamics and are now studied at the single molecule level thanks to scanning probe techniques and synchrotron experiments. Though magnetic hysteresis has been observed on isolated molecules on surface and even enhanced by the interaction with the substrate, cryogenic temperatures are necessary to preserve the magnetic information.

Abstract: Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. They have long been the subject of spectroscopic investigation, but it is only very recently [2,3] that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1s to 2p, due to the difficulty of obtaining short, intense pulses in the relevant wavelength range, around 10THz.

Abstract: A series of fundamental discoveries over the past thirty years has dramatically improved our ability to read, write, and process magnetically stored information. I will briefly review some of these advances before focusing on the recently discovered spin-orbit torques, which act on the collective spin of thin film ferromagnets when they are placed on a substrate with strong spin-orbit interactions and are particularly promising for applications.

Guilherme will present a scalable design for a silicon quantum processor that exploits the electric dipole induced on a donor with a top-gated structure. Quantum information is encoded in either the nuclear-spin or the flip-flop states of electron and nucleus. The physical qubits are spaced by hundreds of nanometers and coupled through direct electric dipole interactions and/or photonic links. They can be controlled at high-speeds by extremely low-power microwave fields, while still preserving their outstanding coherence times.

The scanning tunneling microscope is an amazing experimental tool because of its atomic-scale spatial resolution.
This can be combined with the use of low temperatures, culminating in precise atom manipulation and spectroscopy with microvolt energy resolution. In this talk I will apply these techniques to the investigation of the quantum spin properties of transition metal atoms on surfaces. We will conclude with our recent measurements of electron spin resonance in an STM on individual Fe atoms supported on an insulating thin film.

QuTech, Kavli Institute of Nanoscience, Delft University of Technology, Netherlands

When:

4pm Thursday 8 October 2015

Where:

School of Physics, Room 64

In his seminal work[1], John Bell proved that no theory of nature that obeys locality, realism and free will can reproduce all the predictions of quantum theory. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes.

Photoexcitation above the band gap of insulators or semiconductors may lead to non-equilibrium processes on ultrafast timescales. Depending on excitation density their dynamics are governed by exciton formation and electron-phonon scattering or more complex phenomena leading to phase transitions. These processes typically occur on ultrafast (femto- to picosecond) time scales. We employ femtosecond time- and angle-resolved photoemission spectroscopy (trARPES) to study surface excition formation as well as ultrafast insulator-to-metal (IM) transitions in several materials.

I am an artist with a background in science. The art I make is inspired by scientific theories and thinking, and engaging with scientists across different fields and exploring their ideas is a key catalyst for my work. Having had the pleasure of meeting and talking with several members of this group, I got a sense of the extraordinary work being done here.

Magnetism in thin films can significantly deviate from commonly known magnetic configurations in bulk systems due to low dimensionality, hybridization effects, a change of the lattice constant, stacking and broken inversion symmetry at interfaces. This can lead to non-collinear spin states such as spin spirals or skyrmions. Especially skyrmions offer great potential as information carriers in future robust, high-density, and energy-efficient spintronic devices.

Magnetic resonance is essential in revealing the structure and dynamics of biomolecules. However, measuring the magnetic resonance spectrum of single biomolecules has remained an elusive goal. In this talk, I will introduce our recent results of detecting the electron spin resonance signal from a single spin-labeled protein under ambient conditions. As a sensor, we use a single nitrogen vacancy center in bulk diamond in close proximity to the protein. Some explorations of scallable quantum computation with this technique will also be demonstrated.

The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron’s charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications.

There have been many architectures for quantum computer and quantum information devices proposed, yet we face a gap between these proof-of-principle idea and feasible quantum devices. We focus on an integrated cavity device based on a single diamond NV center to identify the problems and obstacles integrating necessary elements to perform certain tasks within a threshold error.

For GaAs spin qubits all essential operations for QI have been demonstrated and coherence times are increasing. However, coherent control is still, like in many other electron spin qubit systems, impaired by the fluctuating nuclear spin bath of the host material. Previous experiments have shown dynamic nuclear polarization with feedback to significantly prolong de inhomogeneous dephasing time T2∗ by narrowing the distribution of nuclear Overhauser field fluctuations.

Long coherence times and fast manipulation are two desirable qualities of a qubit that for many systems are mutually incompatible. Storing quantum information in an ancillary qubit, i.e. a `quantum memory', is a strategy to address this issue. It is a advantageous property of donor impurities in silicon to have qubits of both qualities in a single lattice site. This talk will present results of the storage and retrieval of quantum information from a single donor electron spin to its host phosphorus nucleus in isotopically-enriched $^{28}$Si.

Key roles of isotope engineering in silicon and diamond quantum information processing are discussed. While removal of the background 29Si nuclear spins is proven crucial for extending the coherence time of spin qubits in silicon, removal of the background 28Si, 29Si, and 30Si mass fluctuations is also shown to be important for defining the nuclear magnetic resonance frequencies of donors such as 31P in silicon. Effects of removing 13C nuclear spins in diamond are also similar.

Compressed sensing techniques have been successfully applied to quantum state tomography, enabling the efficient determination of states that are nearly pure, i.e, of low rank. We show how compressed sensing may be used even when the states to be reconstructed are full rank. Instead, the necessary requirement is that the states be sparse in some known basis (e.g. the Pauli basis). Physical systems at high temperatures in thermal equilibrium are important examples of such states.

Since the first observation, almost 15 years ago, of coherent oscillations in a superconducting qubit there have been significant developments in the field of superconducting quantum circuits. With improvements of coherence times by over 5 order of magnitude, it is now possible to implement simple quantum algorithms with these circuits. In parallel to these developments, much effort has been invested in using superconducting qubits as artificial atoms to explore quantum optics in unconventional parameter regimes.